1. Field of Invention
The present invention relates generally to chemical mechanical polishing systems. More particularly, the present invention relates to a sensor arrangement that allows for an efficient and accurate determination to be made regarding when a chemical mechanical polishing procedure is completed.
2. Description of the Related Art
Ensuring the planarity of the surface of a semiconductor wafer is crucial if the integrity of photolithography processes performed on the semiconductor wafer is to be maintained at a high level. That is, it is important that the surface of a semiconductor wafer be planar in order to meet the requirements of photolithography processes. By way of example, the planarity of the surface of a semiconductor wafer is critical to photolithography processes as the depth of focus of photolithography processes may not be adequate for surfaces which do not have a consistent height.
A chemical mechanical polishing (CMP) process is often used to planarize the surface of a semiconductor wafer. CMP is effective in improving the global planarity of the surface of a semiconductor wafer. The assurance of planarity is crucial to the lithography process as the depth of focus of the lithography process is often inadequate for surfaces which do not have a consistent height.
CMP processes generally utilize a polishing pad made from a synthetic fabric and a polishing slurry which includes pH-balanced chemicals, such as sodium hydroxide, and silicon dioxide particles. A semiconductor wafer is mounted on a polishing fixture such that the wafer is pressed against the polishing pad under pressure. The fixture then rotates and translates the wafer relative to the polishing pad. The polishing slurry assists in the actual polishing of the wafer. Abrasive forces are created by the motion of a wafer against a polishing pad and cause material to be abraded away from the surface of the wafer. While the pH of the polishing slurry controls chemical reactions such as the oxidation of the chemicals which comprise an insulating layer of the wafer, the size of the silicon dioxide particles in the polishing slurry controls the physical abrasion of surface of the wafer. The polishing of the wafer is accomplished when abrasive forces enable the silicon dioxide particles to abrade away the oxidized chemicals. Often, different layers of the wafer may be thinned to a desired thickness through CMP.
In general, it is desirable to detect when the surface of a wafer has been polished to a desired level, e.g., when the surface of a wafer is planar. To determine whether the wafer surface has reached a desired level of planarity or, more generally, to determine when a polishing endpoint is reached, the wafer may be removed from an overall CMP apparatus and inspected. With such an approach, if the wafer does not meet desired specifications, it may be necessary to reload the wafer onto the overall CMP apparatus and continue polishing the wafer. Such an approach, while often effective, is inefficient in that it is both time consuming and relatively labor-intensive. In addition, since such an approach is not in-situ, there may be occasions in which excess material is removed from the surface of a wafer before the wafer is inspected. When excess material is removed, the wafer may be deemed unusable.
To determine when a desired film thickness is reached or, more generally, to determine when a polishing endpoint has been reached, some CMP systems utilize windows embedded in a polishing pad to allow the surface of a wafer to effectively be viewed in-situ. With reference to FIGS. 1A and 1B, one conventional CMP system will be described. FIG. 1A is a diagrammatic top-view representation of a CMP polishing system 100, and FIG. 1B is a diagrammatic side-view representation of CMP polishing system 100. A CMP polishing system 100 includes a polishing pad or platen 104 attached to an actuator assembly (not shown) at an annulus 108. As polishing pad 104 rotates about a z-direction 110 while making contact with a wafer 114 that is being polished. Wafer 114 also rotates about x-direction 110.
A window 116 that is embedded in polishing pad 104 effectively moves with polishing pad 104 and, when wafer 114 is positioned directly below window 116, a sensing system which “views” wafer 114 through window 116 may effectively sense the status of a polishing process in-situ. Such a sensing system may utilize a laser interferometer or an electrical eddy current sensor to measure the thickness of a layer of wafer 114, or to determine whether the surface of wafer 114 is suitably planar. That is, a sensing system (not shown) effectively views wafer 114 through window 116 and allows a determination to be made as to whether a polishing endpoint has been reached.
While the use of window 116 in conjunction with a sensing system (not shown) is effective in allowing a polishing endpoint to be detected in-situ, the alignment of window 116 often needs to be readjusted, as contact of window 116 with wafer 114 and abrasive forces between window 116 and wafer 114 may adversely affect the alignment of window 116. Additionally, window 116 may need to be replaced or changed out fairly often, as window 116 is effectively polished by a CMP process. When window 116 is polished, the transparency of window 116 may be adversely affected, thereby affective the performance of a sensing system (not shown) that utilizes window 116. Realigning and replacing window 116 within polishing pad 104 may be a time-consuming process. Further, each time window 116 needs to be realigned or replaced, polishing pad 104 must effectively be taken off-line and not used for polishing purposes until after window 116 is sufficiently realigned or replaced.
In addition, window 116 transits on and off of wafer 114 during a CMP process. Hence, wafer 114 may not be viewed during all portions of a polishing process. The inability to view wafer 114 during all portions of a polishing process may result in an endpoint not being detected until the endpoint has been passed. In other words, if wafer 114 is not viewed throughout the polishing process, there is a possibility that an endpoint may be reached during the time in which window 116 is not positioned over wafer 114.
Therefore, what is needed is a method and an apparatus that allows a polishing endpoint to be efficiently detected. That is, what is desired is a system that enables a polishing endpoint to be efficiently detected in-situ by monitoring a wafer surface throughout a polishing process.